1. Field of the Invention
The present invention relates to a method of adjusting a gas sensor of the type, such as the "diffusion limited current" system, which measures the concentration of a gas of interest in an ambient atmosphere by employing the effect of limited gas diffusion into an analyzing compartment.
2. Background of the Invention
Techniques for achieving precise measurement of the concentration of gases in an ambient atmosphere are in great demand in areas of pollution control and for various industrial purposes. Two of the gas sensors that have been developed in order to attain this end are as follows. In one type, which is generally referred to as the "diffusion limited current" system, a solid electrolyte capable of conducting ions of a selected gaseous component is provided with a pair of electrodes to fabricate a pump element which is placed in contact with an analyzing compartment having a porous material that is capable of limiting gas diffusion. When a voltage is impressed on the pump element, a gaseous component of interest will be discharged out of the analyzing compartment and the concentration of that gas in the ambient atmosphere is measured in terms of the diffusion limited current flowing through the electrolyte. In the other type of gas sensor, either a concentration electrochemical cell having the same construction as the pump element or a gas sensing element is placed in contact with the analyzing compartment and the current flowing through the pump element is adjusted such that the electrochemical cell or gas sensing element will produce a constant output, and the concentration of a gas of interest in the ambient atmosphere is measured in terms of the adjusted current.
The gas concentration vs. output characteristics of the gas sensors described above are determined by the porosity of the porous body provided in the analyzing compartment, or its capability of limiting gas diffusion into that compartment. However, the porosity of the porous body is very subtle in nature and it has been impossible to achieve consistent production of gas sensors featuring uniform limitation of gas diffusion or postmanufacture adjustment of this parameter. Therefore, in order to achieve precise measurements of the concentration of a gas of interest, it has been necessary to adjust individual measuring circuits in accordance with the characteristics of the gas sensor employed.